The present invention relates to the deposition of dielectric layers during wafer processing and more specifically to a method and apparatus for forming a fluorine doped layer having a low dielectric constant and good gap-filling capability.
One of the primary steps in the fabrication of modern semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of gases. Such a deposition process is referred to as chemical vapor deposition or "CVD". Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions take place to produce a desired film.
Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Today's wafer fabrication plants are routinely producing 0.5 and even 0.35 micron feature size devices, and tomorrow's plants soon will be producing devices having even smaller geometries.
As device sizes become smaller and integration density increases, issues that were not previously considered important by the industry are becoming of paramount concern. With the advent of multilevel metal technology in which three, four, or more layers of metal are formed on the semiconductors, one goal of semiconductor manufacturers is lowering the dielectric constant of insulating layers such as intermetal dielectric layers deposited by thermal CVD or plasma enhanced CVD (PECVD) methods. Low dielectric constant films are particularly desirable for intermetal dielectric (IMD) layers to reduce the RC time delay of the interconnect metallization, to prevent cross talk between the different levels of metallization, and to reduce device power consumption.
Many approaches to obtain lower dielectric constants have been proposed. One of the more promising solutions is the incorporation of fluorine or other halogen elements, such as chlorine or bromine, into a silicon oxide layer to form an oxide/halogen network. An example of halogen incorporation is described in U.S. patent application Ser. No. 08/344,283, commonly assigned to Applied Materials, Inc., filed on Nov. 24, 1994. Fluorine, the preferred halogen dopant for silicon oxide films, lowers the dielectric constant of the silicon oxide film because fluorine is an electronegative atom that decreases the polarizability of the overall Si--O--F network. Fluorine-doped silicon oxide films are also referred to as fluorosilicate glass films or FSG for short.
In addition to decreasing the dielectric constant, incorporating fluorine in intermetal silicon oxide layers also helps solve common problems encountered in fabricating smaller geometry devices, such as filling closely spaced gaps on semiconductor structures. It is believed that because fluorine is an etching species, fluorine doping introduces a deposition/etch/deposition effect on oxide formation. The deposition/etch/deposition effect allows FSG films to have improved gap filling capabilities such that the films are able to adequately cover adjacent metal layers having an aspect ratio of 1.8 or more.
Thus, manufacturers desire to include fluorine in various dielectric layers and particularly in intermetal dielectric layers. A variety of different precursor gases and liquids have been employed as the source of fluorine in the formation of these FSG films. Some of these precursors include NF.sub.3, HF, SF.sub.6, CF.sub.4, C.sub.2 F.sub.6, C.sub.2 Cl.sub.3 F.sub.3 and triethoxyfluorosilane (TEFS) among others.
SiF.sub.4 has been used in conjunction with some of the fluorine sources listed above to form FSG films. In such processes, however, SiF.sub.4 has been employed as a silicon source and a second gas, e.g., NF.sub.3, HF, SF.sub.6, CF.sub.4, C.sub.2 F.sub.6, C.sub.2 Cl.sub.3 F.sub.3, SF.sub.6 or F.sub.2, has been used as a source of fluorine. SiF.sub.4 has not been used as the only source of fluorine for the formation of FSG films in combination with a silicon-containing source such as an organic silicon source like tetraethylorthosilane (TEOS).